(Internal) Depth of Field Not Getting Right Spot
I suggest you don't use the defocus node. You can just select the camera, go to its settings, hit the high quality button and set the f-stop level down to like 0.4 to see very precise where you are focusing. Then, set a random focus distance in the box next to the f-stop setting. Go in the 3d view, hit 'N' and check the 'Depth of field' box under 'Shading'. Now you can see the focus distance in the viewport as it is going to be rendered. Now start playing with the distance in the camera settings. Also, you can set the F-stop back to your preference after you are sure of the distance.